1. Technical Field
The present invention relates to a method of manufacturing an optical fiber base material being capable of manufacturing an optical fiber base material of high quality by so-called VAD.
2. Related Art
VAD is well-known as a method of manufacturing base materials for optical fibers. This method employs the following apparatus, for example.
In this apparatus, glass particles produced with a core deposition burner and a cladding deposition burner disposed in a reaction chamber; and the glass particles are deposited onto a tip of a starter mounted on a shaft which rotatably lifts up, so that a porous glass base material for optical fiber made of a core layer and a cladding layer is manufactured. The core layer may be SiO2 with which GeO2 is doped, and the cladding layer may be substantially pure SiO2.
The porous glass base material 1 manufactured as described above is dehydrated and sintered in a heating furnace. The heating furnace has a furnace tube 2 which can be sealed, an electric furnace 3 which heats a part of or the whole of the furnace tube 2, a gas introducing port 4 which introduces any gas into the furnace tube and a gas discharging port 5 which discharges the exhaust gas as shown in FIG. 1, for example. FIGS. 1A to 1C progressively show vitrifying the porous glass base material. Here, reference numeral 6 indicates a shaft which supports the porous glass base material 1.
Dehydrating is performed by heating the base material at approximately 1,100 degrees Celsius in dehydrating gas composed of such as chlorine, oxygen and helium. Meanwhile, vitrifying is performed by heating the base material at approximately 1,500 degrees Celsius in an atmosphere containing such as helium.
For the furnace tube forming a part of the heating furnace, conventionally a silica tube made of natural quartz has been employed as described in Japanese Patent Application Publication No. 2004-002109. For example, the silica tube may be an electric-furnace-melted natural quartz glass tube such as HERALUX-E (trade name), available from Shin-Etsu Quartz Products Co., Ltd., which is made by pulverizing natural quartz and melting in an electric furnace (herein after referred to as a natural quartz glass tube).
When a porous glass base material is dehydrated and sintered by using the natural quartz glass tube as a furnace tube, a resultant optical fiber base material has a problem that the transmission loss increases possibly caused by a little impurity contained in natural quartz. Therefore, the natural quartz furnace tube contains impurities and crystallites, and they nucleate for crystallization. Then, crystallization (into cristobalite) progresses under a high temperature. Particularly, the electric-furnace-melted natural quartz contains aluminum equal to or more than 15 ppm, and the aluminum acts as the core of crystallization. In this case, the impurities diffuse along the crystal grain boundary and are easily discharged into the furnace tube to contaminate the optical fiber base material.
In order to address the above described problem, the inventor proposed that silicide as a raw material is hydrolyzed with oxyhydrogen flame, and a resultant synthetic quartz is used as a furnace tube. Since synthetic quartz contains little impurity and crystallite, crystallization does not progress, so that an advantage of synthetic quartz is that it is much less likely to contaminate the optical fiber base material with any impurity.
An optical fiber base material manufactured by the above described method may be formed as a finished optical fiber base material by adding a cladding to the periphery thereof.
The problem that the transmission loss of the optical fiber base material increases when the natural quartz glass tube is used as the furnace tube can be solved by using the synthetic quartz glass tube in place of the natural quartz glass tube. However, there is another problem that the strength of the synthetic quartz glass tube at a high temperature is less than that of the natural quartz glass tube. For example, since the synthetic quartz furnace tube does not crystallize even if it is used at a high temperature. Therefore, the synthetic quartz furnace tube softens around 1,500 degrees Celsius within processing temperatures, and the furnace tube is deformed because the pressure in the furnace tube is slightly fluctuated around ±3 kPa. This deformation may cause the furnace tube to contact and damage the porous glass base material. Meanwhile, the natural quartz furnace tube easily crystallizes as described above, and the crystallized furnace tube is not easily deformed at a high temperature.